The present invention relates to water-soluble degradable synthetic vinyl polymers and methods of use thereof in subterranean applications. More specifically, at least in some embodiments, the present invention relates to water-soluble degradable synthetic vinyl polymers having at least one labile group in the backbone of the polymer, and methods of use thereof in subterranean applications.
Water-soluble polymers are used in a wide range of industries and products. For example, they may be used as rheology modifiers, stabilizers, and emulsifiers in a variety of products. They are also used in detergents, shampoos, food products, skin lotions, textiles, paints and in the pharmaceutical and oil industry as viscosifiers, flocculants, drag reducing agents, or mobility control fluids. Additionally, water-soluble polymers play an important role in the production of oil and gas. They are used in treatment fluids in various applications such as fracturing, drilling, completion, and work-over applications. Oftentimes, such water-soluble polymers are used as viscosifiers in such application.
One problem associated with the use of synthetic polymers is that they generally are stable in the environment, and thus, may be retained in water or in the ground for many years. This is a particularly troublesome aspect of using such polymers in subterranean applications, especially because of the high molecular weight of the polymers that are typically used. For example, in subterranean applications, adsorption and accumulation of such polymers on mineral surfaces within the formation can lead to the entrapment and formation of undesirable deposits within the formation, which can negatively impact the permeability and conductivity of the formation. Moreover, oftentimes a follow-up expensive clean-up operation is often required to combat this.
In subterranean applications, there is a need for polymers of high molecular weight that can be degraded to smaller molecular weights for a number of applications, including hydraulic fracturing, gravel packing, “frac-packing,” fluid loss pills, diverting particles, viscous sweeps, work-over fluids, drilling fluids, rheological modifiers, and so forth. Generally, these subterranean treatment fluids comprise viscosifying agents that comprise natural polysaccharides such as guar, cellulose, xanthan, and the like or water-soluble polymers synthetic polymers that have hydrocarbon backbones, which are generally thought to not be degradable due to their resistance to hydrolysis, oxidative cleavage, temperature or enzymatic attack.
In these subterranean applications, it is preferable for the polymer to be removed from the formation after its use has been exploited. For example, in hydraulic fracturing applications, aqueous crosslinked gels that are generally prepared from viscosifying agents are used to fracture formations and transport proppant into those fractures. After placement of the proppant in the fracture(s), it is preferable for the polymer that made up the crosslinked gel to be broken in some way for recovery of a lower viscosity fluid. Using oxidative breakers or enzymes is a common method that is used to break such polymers to reduce the viscosity of the fluid for recovery.
There are several drawbacks to such methods. Oxidative breakers may be dissolved in the fluid, but may be lost due to fluid loss as the gel loses water into the porous oil-containing rock of the formation. To circumvent this type of problem, an excess of the oxidative breakers may be used or a fluid loss control agent may be used, which may not be desirable. In some instances, the addition of an oxidative breaker may prematurely decrease the viscosity of the fluid, and thus more polymer may be needed to transport the proppants (which is undesirable). Enzymes are specific to the substrates in which they are effective and there is a diffusion limitation on the movement of enzymes through a crosslinked gel system. Enzymes also have a narrow temperature and pH range where they are effective. Enzymes lose their activity as temperature is raised and most of the enzymes are ineffective above 60° C. Enzymes are also ineffective at extreme pH values and oftentimes work best under neutral conditions. Most of the fluids used in oil-field applications have a pH of 8 and above where the effectiveness of enzymes is low. Furthermore, oxidative breakers may be dissolved in the fluid, but may be lost due to fluid loss as the gel loses water into the porous oil-containing rock of the formation.
To circumvent this type of problem, an excess of oxidants may be used or a fluid loss control agent may be used. The use of enzymes and oxidants and enzymes may not guarantee the complete degradation of the polymer system, irrespective of the use of additional fluid loss control agents. The incomplete degradation of the polymer system used can lead to deposition of polymeric materials onto the oil-bearing rock surfaces within the formation, eventually impeding production. Additionally, an incomplete degradation can lead to an ineffective reduction in the viscosity of the fracturing fluid to the level needed to deposit the proppant and return the fluid back to the surface.